(function (doc, win) {
   var docEl = doc.documentElement,
        resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
        recalc    = function () {
               var clientWidth = docEl.clientWidth;
               if (clientWidth>=640) {
                  clientWidth = 640;
               };
               if (!clientWidth) return;
             docEl.style.fontSize = 100 * (clientWidth / 640) + 'px';
        };
        if (!doc.addEventListener) return;
        win.addEventListener(resizeEvt, recalc, false);
        doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);